OVERALL ? SUMMARY/ABSTRACT Organ transplantation remains a mainstay therapeutic strategy for patients with end organ diseases. One of the highest unmet needs to improve long-term transplant outcomes is devising more effective immune modulation. This requires innovative mechanistic studies of transplant alloimmunity. The lymph node (LN) is the quintessential organ of alloimmunity. While the recognition of alloantigens in the LN is fundamental to the generation of alloreactive T cells, our groups also have shown that the LN plays an important role in alloimmune-regulation and Treg-mediated tolerance. These multifaceted functions rest on the nature of LNs as extremely specialized organs with unique microvasculature, stromal fibers, and stromal cells (referred to as fibroblastic reticular cells [FRCs]). Our overarching hypothesis is that manipulating the microenvironment of LNs will provide a unique opportunity to direct the alloimmune reaction towards an anti-inflammatory tolerance response. Our major goals are to understand the cellular and molecular mechanisms that govern the microanatomical adaptation of the LN during immune activation or tolerance induction, and to develop highly innovative therapeutic strategies that promote a regulatory LN microenvironment and result in immune tolerance. This PPG sets forth a platform for connecting two teams (Drs. Abdi and Bromberg) with complementary skills and expertise in LN alloimmune-biology. Project 1 will test the hypothesis that sustained activation of FRCs of the LN during alloimmunity will result in FRC transformation to proinflammatory myofibroblasts creating an inflammatory milieu within the LN, which would further promote alloimmunity. Our corollary hypothesis is that restoration of the function of FRCs and microanatomy of the LNs will enhance their immunoregulatory function and promote tolerance. Aim 1 will examine the role of the HVEM/LIGHT pathway in the differentiation of FRCs into proinflammatory myofibroblasts, thereby creating an inflammatory milieu within the LN microenvironment and promoting transplant immunity. Aim 2 will investigate the mechanisms by which fibrotic FRCs promote a pro-inflammatory response in the LN. Aim 3 will reprogram the stroma of LNs via FRC delivery or LN-targeted delivery of senescence inhibitors to further promote alloimmune tolerance. Project 2 will test the hypothesis that FRCs regulate the LN laminin ?4:?5 (LAMA4/LAMA5) ratio and control the fate of the immune response. Aim 1 will define the role of stromal cells in controlling the balance of LAMA4 and LAMA5. Aim 2 will define the role of LT?R as a key pathway in regulating the formation of LAMA5. Aim 3 will use targeted delivery of anti-CD40L and anti-LAMA5 mAbs to the LN to promote tolerance. An Administrative Core (Core A) and Nanoparticle and FRC Core (Core B) will provide the infrastructure and resources to support these two projects. The ultimate goal of these well-integrated and highly synergistic Projects and Cores is to generate transformative mechanistic data, which will lay the groundwork for developing highly targeted and innovative therapeutic strategies for transplantation.